Many different types of sonar apparatus are known for locating features that lie on the underwater floor in front of a boat and displaying those features to a user on a display.
The known types of sonar apparatus generally fall into three categories.
A first category comprises phased arrays, in which a multitude of receivers are connected to a processor via variable delay lines. The processor adjusts the delays on the delay lines until the signals from each receiver produced by sound waves that have been reflected from points on the underwater floor are in phase. By measuring the time delay on each delay line, the direction of arrival of the sound waves, and hence the direction of the reflecting feature on the underwater floor, can be determined. By transmitting sound waves in a wide arc in front of the boat and detecting the reflected sound waves using the phased array, the positions of features on the underwater floor in the arc in front of the boat can be determined and displayed. However, apparatus of this type suffer from the problem that the phased array occupies a large physical area, and therefore requires a large hole to be drilled in the bottom of the boat to fit the apparatus. Furthermore, apparatus of this type require a long time to gather sufficient data to produce a reasonable image of the underwater floor, as each point on the floor has to be considered individually. Alternatively, to reduce this time, a large amount of parallel processing is performed, but this increases the cost of the apparatus significantly. These problems preclude the use of such apparatus on small boats.
A second category of apparatus comprises apparatus which employ a “staring array” of transducers, which is fixed in one direction to transmit sound waves into and receive sound waves from a narrow segment of water directly in front of the boat (hence the name “staring array” as the array stares in one direction). Apparatus of this type detect the phase of the reflected sound waves by processing the signals generated by different pairs of the transducers in the array. These apparatus have the advantage of a small physical size, high reliability and low cost. However, they suffer from the problem that points on the underwater floor are only detected in a narrow segment of water so that a full display of the underwater floor in front of the boat cannot be provided to the user.
A third category of apparatus comprise apparatus which employ an array of transmitting and receiving transducers, and which physically rotate the array to scan a wide arc in front of the boat so as to transmit sound waves into, and receive reflected sound waves from, that arc. As with phased arrays, apparatus of this type enable features on the underwater floor in front of the boat to be located within a wide arc. However, apparatus of this type suffer from the problem that a mechanical mechanism is necessary to rotate the array of transducers, and this mechanical mechanism has a tendency to break due to the continued scan movement. Furthermore, accuracy is reduced because of losses and beam angle scattering due to acoustic coupling from the moving array to the water.